The present invention relates to solid state power amplifier modules. In particular, the invention relates to a solid state power amplifier module that splits a signal into multiple parts, uses distributed amplifiers to amplify the parts, and recombines the amplified parts into a single output.
Solid state power amplifier modules (SSPAs) have a variety of uses. For example, SSPAs may be used in satellites to amplify severely attenuated ground transmissions to a level suitable for processing in the satellite. SSPAs may also be used to perform the necessary amplification for signals transmitted to other satellites in a crosslink application, or to the earth for reception by ground based receivers.
Typical SSPAs achieve signal amplification levels of over 12 db. Because a single amplifier chip cannot achieve this level of power gain without introducing excessive noise into the signal and without incurring excessive size and power consumption, modern SSPA designs use a radial splitting and combining architecture in which the signal is divided into numerous individual parts. The individual parts are then individually amplified by an equal number of amplifiers. Finally, the outputs of the amplifiers are combined into a single output which achieves the desired overall signal amplification.
One difficulty faced by previous SSPA designs is that they do not work well at frequencies above a few gigahertz (GHz). Parasitic effects of interconnections, splitter and combiner structure, and the materials used to propagate the signal all contribute to the frequency limitations inherent with prior SSPAs. Interest continues to grow, however, in the communications industry on signals operating at frequencies much higher than a few GHz.
U.S. Pat. No. 5,218,322 to Allison et al. discloses a solid state microwave power amplifier module. Allison uses a first substrate formed of a low temperature co-fired ceramic material. The first substrate includes a radial power splitter that divides an input signal into a number of radially extending transmission lines placed in the substrate and terminating in respective output ends. Allison provides a second substrate including a number of solid state power amplifiers and transmission line circuitry for connecting the respective output ends to inputs of the solid state power amplifiers. The second substrate also includes a radial power combiner that combines the outputs of the solid state power amplifiers. The first substrate and the second substrate are joined such that the divider output signals are connected through vertical coaxial transmission lines to corresponding transmission lines in the combiner substrate.
In the Allison device, the radially extending transmission lines in the divider are created with stripline transmission lines (formed as a conductor between two ground planes and necessitating a multi-layer divider). Furthermore, at the edge of the divider, vertical coaxial transmission lines are created as metal filled vias surrounding a center conductor. The vertical coaxial transmission lines connect the radially extending transmission lines to the combiner. The combiner in Allison uses microstrip conductors coupled to the vertical coaxial transmission lines to connect the radial splitter transmission lines to the amplifier inputs on the combiner and to connect the amplifier outputs to the subsequent combiner structure.
The SSPA in Allison, however, is generally unsuitable for signals above a few GHz in frequency. Because the striplines, microstrips, and vertical coaxial structures all include parasitic effects (for example, self inductance), higher frequency signals tend to be severely attenuated when passing through the splitter and combiner structures. The parasitic effects are increased by the complicated multilayer interconnections required between the striplines, vertical coaxial transmission lines, and the microstrips.
In addition, previous SSPA designs tend to be bulky and heavy. The size and weight of the SSPA reduces the amount of other electronics a satellite can carry and provide power for, and increases the size and cost of the launch vehicle used put the satellite into orbit. Present SSPA designs, for example, include air dielectric waveguides with large flanges. The amplifier modules are separately built and later assembled with the splitter and combiner. The individual pieces of the SSPA require complex machining and, typically, include a large number of components that must be manually assembled. The resulting SSPA not only has excessive weight, but also has a high manufacturing cost and is generally limited to use at low frequency.
In part, interest in higher signal frequencies is a natural consequence of the lower frequency bands already operating at capacity to provide communications services. In addition, with fewer governmental restrictions being imposed on the availability of extremely high frequency bands (for example frequency bands extending over the 10-100 GHz range), those frequency bands are being turned to to provide bandwidth for additional communications services. An SSPA design able to operate at much higher frequencies is required to take advantage of the bandwidth available in the 10-100 GHz range.
Therefore a need is present in the industry for an improved high frequency SSPA module which overcomes the disadvantages discussed above and previously experienced.